Persistent luminescence (PL), also called afterglow or long-lasting phosphorescence, is the phenomenon encountered in materials that glow well after the end of an excitation with UV or visible light. PL materials have been used in a wide variety of applications, such as luminous dials and displays, fiber-optic thermometers, and forensic and military identification materials. While the mechanism underlying PL is not fully understood, it is generally agreed that the phenomenon involves energy traps in a material that are filled during excitation. After excitation, the stored energy is gradually released to emitter centers.
In near infrared (NIR) persistent luminescence, PL phosphors can store excitation energy in energy traps while continuing to emit photons for weeks after excitation ceases. (Pan, et al. 2012 Nat Mater 11, 58; Abdukayum, et al. 2013 J Am Chem Soc 135, 14125; Liu, et al. 2013 Sci Rep-Uk 3, 1554.) Studies have been done to explore advanced biomedical applications for NIR persistent luminescence nanoparticles (PLNPs) that can emit PL in the optical bio-imaging window (˜650-1000 nm) for hours, or even days, after cessation of excitation. (Pan, et al. 2012 Nat Mater 11, 58; Maldiney, et al. 2014 Nat Mater 13, 418.) The temporal separation of excitation and afterglow properties of these persistent phosphors makes them ideal as in vivo optical imaging contrast reagents. (Chermont, et al. 2007 P Natl Acad Sci USA 104, 9266; Maldiney, et al. 2011 J Am Chem Soc 133, 11810; Maldiney, et al. 2011 Acs Nano 5, 854.) Excitation resource and relevant complicated optics that are necessary for traditional fluorescence imaging are no longer needed.
Until now, persistent luminescence has relied on short-wavelength excitation (e.g., ultraviolet light), which has rather limited tissue-penetration depth. (Clabau, et al. 2005 Chem Mater 17, 3904; Lin, et al. 2001 J Mater Sci Lett 20, 1505; Rodrigues, et al. 2014 J Mater Chem C 2, 1612; Rodrigues, et al. 2012 J Phys Chem C 116, 11232.) To address this problem, a NIR-light-stimulated PL mechanism was proposed in LiGa5O8:Cr3+, to release energy trapped in deeper energy levels of the phosphor. In this case, however, the energy must be pre-charged by UV-light and the photo-stimulated emission continues to weaken after each cycle of photo-stimulation and finally becomes extinguished. (Liu, et al. 2013 Sci Rep-Uk 3, 1554; Zhuang, et al. 2013 J Mater Chem C 1, 7849.) Very recently, the PL phosphor, ZnGa2O4:Cr3+ (ZGC), was found to be activatable using tissue-penetrable red light, which means that energy can be recharged and NIR PL imaging is no longer limited by the luminescence-decay life-time of the phosphor. (Maldiney, et al. 2014 Nat Mater 13, 418.) Thus ZGC is arguably the optimal rechargeable NIR persistent emitting phosphor reported to date.
Despite such inspiring progress, production of uniformly structured NIR PL ZGC phosphors remains challenging. Advances in the development of PLNPs for both basic research and commercialization have been hampered by their complicated synthesis methods. To make such NIR-persistent phosphors bulk crystal requires temperatures greater than 750° C. in traditional solid-state annealing reactions. (Pan, et al. 2012 Nat Mater 11, 58; Clabau, et al. 2005 Chem Mater 17, 3904; Setlur, et al. 2008 J Appl Phys 103, 053513.) Moreover, to convert such bulk crystal into nanoparticles that are sufficiently disperse for biological applications, certain tedious physical treatments such as grinding or laser ablation must be utilized. (Abdukayum, et al. 2013 J Am Chem Soc 135, 14125; Liu, et al. 2013 Sci Rep-Uk 3, 1554; Maldiney, et al. 2014 Adv Funct Mater DOI 10.1002/adfm.201401612; Maldiney, et al. 2014 Nanoscale 6 (22), 13970-13976; Maldiney, et al. 2012 Opt Mater Express 2, 261.) The afforded products are generally highly heterogeneous and suffer from severe agglomeration. In addition, bio-imaging applications generally require that the nanocrystals be biocompatible, which means that the PLNPs need to be comparable in size to the biomolecules they label, so as not to interfere with cellular systems.
Thus, novel and improved synthetic methodologies, in particular aqueous-phase chemical synthesis for sub-10 nm NIR PLNPs that are uniform and can be homogeneously dispersed in a carrying medium (e.g., aqueous solution), are strongly desired.